The present invention relates generally to systems for demodulating angle-modulated signals, and more particularly to a system using a tracking loop for demodulating angle-modulated signals, with a large signal to noise ratio.
A system which uses a phase locked loop (hereinafter referred to as "PLL") comprises a phase comparator, a loop filter, a DC amplifier, a voltage-controlled oscillator, and other components. Heretofore, such a system has been used for demodulating angle-modulated wave signals.
In this PLL, if a wide band width of the loop is used, noise will readily impart an effect. However, there is no ready effect from noise, if a narrow band width of the loop is used. The loop band width of a PLL, however, cannot be narrower than a certain degree, since it depends upon factors such as the frequency band width of the angle-modulated wave signal and the modulation signal level. Consequently, if there is a high level of noise within or in the vicinity of the frequency band of the angle-modulated wave signal, the noise will impart an effect. In some cases, the phase locked loop may unlock. In such a case, the loop may lock with the noise component, and normal demodulation becomes impossible, whereby an abnormal noise is generated.
Furthermore, as is known, the input signal frequency versus error signal voltage characteristic of a PLL has a linearly sloped characteristic. The error voltage increases as the frequency increases from a center frequency f0 and decreases as the frequency decreases from this center frequency f0. Thus, as the frequency is gradually elevated from the center frequency f0, the error voltage also increases (with positive polarity) in proportion thereto. However, when the frequency reaches a certain value f1, the error voltage instantaneously becomes zero. As the frequency is gradually lowered from the frequency f1 to a value f2 (f0&lt;f2&lt;f1), the error voltage rises abruptly. The characteristic returns to the original sloped characteristic.
Similarly, when the frequency is gradually lowered from the center frequency f0, the error voltage also decreases (with negative polarity) in proportion thereto. When the frequency reaches a certain value f4, the error voltage instantaneously becomes zero. When the frequency is gradually raised from the value f4 to a value f3 (f4&lt;f3&lt;f0), the error voltage abruptly rises. The characteristic returns to the original sloped characteristic. In this connection, the frequency interval of from f4 to f1 is generally called the "lock range". The frequency range of from f3 to f2 is called the "capture range".
Accordingly, there is no problem whatsoever if the frequency deviation width .DELTA.f of the PLL input angle-modulated wave signal falls between the frequencies f4 to f1, that is, within the lock range. However, if this frequency deviation width is greater than the lock range, the error voltage abruptly becomes zero when the angle-modulated carrier wave exceeds the lock range. Consequently, large abnormal noises are generated in the demodulation output at this time.
Furthermore, if an analog multiplier is used in the phase comparator of the PLL, the loop gain varies in accordance with the magnitude of the level of the angle-modulated wave signal fed as input into the phase comparator. Then, the lock range also varies.
When an interference arises as a consequence of the angle-modulated wave signal and a noise is introduced from the outside and existing within the band of this angle-modulated wave signal, the level of this signal sometimes drops abruptly or becomes zero. The lock range also drops each time this occurs. As a consequence, an unlocking of the PLL occurs frequently, even when the frequency deviation width .DELTA.f of the angle-modulated wave signal is small. Abnormal noise and distortion are generated in the demodulated output.
On one hand, discrete 4-channel system record discs have been described by U.S. Pat. Nos. 3,686,471 and 3,883,699 and have been reduced to practice. In a system of this character, two sum and difference signals are obtained by matrixing 4-channel signals. The difference signals are angle modulated. Direct-wave sum signals and angle-modulated wave difference signals are multiplexed and recorded in a single sound groove of the record disc. When this record disc is reproduced (played) by a reproducing stylus, a tracing distortion is produced because the extreme tip of the reproducing stylus has a certain finite radius, as is known. When this tracing distortion is present, the higher harmonic component of the direct-wave signal becomes admixed into the angle-modulated wave band. The PLL then becomes erroneously locked, in some cases, to this higher harmonic component of the direct-wave signal.
Thus, when a multichannel record disc is being reproduced and a conventional demodulation system having a PLL is used, various problems arise, such as the generation of distortion and abnormal noise in the reproduced signal. These undesirable effects are due to causes such as: erroneous locking with respect to higher harmonic components of the direct-wave signal due to the above mentioned tracing distortion; a drop in the level of the angle-modulated wave signal due to interference imparted to the angle-modulated wave signal by the higher harmonic component; and a drop in the level of the reproduced angle-modulated wave signal due to causes such as a deterioration of the tracing ability of the reproducing stylus, and wear of the sound groove of the record disc.